The present invention relates to the detection of particulates in a fluid, and more particularly, to the measurement of the size distribution of fibers in a fluid sample.
In papermaking and other processes of a similar nature, a dispersion of fibers in a carrier liquid is deposited on webs or the like for undergoing various consolidation, drying, and perhaps coating operations, eventually to emerge as a finished sheet product, i.e., paper. The manufacture of paper spans a variety of specifications as to weight, surface texture, and other physical and chemical criteria to be satisfied. As is well known, the quality of the manufactured paper is very strongly dependent on the quality of the fibers. This quality depends in part on the source of the fibers, and on the manner in which the fibers were refined prior to introduction into the paper manufacturing equipment. An indicator of refining quality is the size distribution of the refined fibers.
In papermaking plants, it is desired that the machinery be adjusted, or controlled, in response to the deviation of the fiber size distribution relative to the targets associated with the particular type of paper manufactured during a given run. To date, on-line equipment for measuring or analyzing the size distribution of fibers, has been very expensive and susceptible to plugging.
Traditionally, "accurate" fiber size distribution has been determined by flowing a diluted sample through various tanks and meshes, on which the fibers are physically classified into four or five size intervals. The deposits on each mesh must be dried and weighed. As a practical matter, this procedure cannot be carried out on line, because the steps of precisely diluting the samples, weighing the meshes before and after classification, and the like, can only be performed in a laboratory or a well-equipped room away from the actual paper manufacturing equipment. The standard types of procedures are generally referred to as either the Clark method or the Bauer-McNett method, and are more fully described in TAPPI Standard T233SU-64 (1964).
More recent attempts of on-line measurement have utilized an optic counting technique, whereby a diluted sample of the fluid is passed through a transparent tube, where a standing light beam is interrupted commensurate with the length of the fiber passing therethrough. It is believed that one type of optic analyzer, available in two models, utilizes measuring tubes having diameters of about 0.2 mm and 0.4 mm, and counting rates of 50 and 100 fibers per second, respectively. Since relatively large fibers, or the occasional agglomeration of fibers, better known as shives, may have a diameter on the order of 0.2 mm to 0.4 mm, this type of equipment is prone to plugging by oversize fibers or shives. When plugging occurs, a backwash or vacuum assist operation is performed to clear the tubes and resume analysis.
Another type of optic analyzer has a generally square flow tube on the order of at least 8.0 mm per side. In this arrangement, the flow area is large enough to avoid blockage by oversized fibers and shives, but it is so large that two light beams projecting perpendicularly to each other, must be used to assure that the fibers can be characterized. Precision is lost and, as a result, the fibers are classified only into only a few categories, e.g., relatively large, average, or relatively small.